I really dont think its the temperature. My Radeon 9800SE runs at 140 F normally and it works fine, its supposed to run kinda hot, although its better to keep it cool. And it obviously isn't the drivers. I've having a problem with my card when I unlock the pipelines so... the only thing I can think of is memory. I want you to try doing this. Go into your BIOS. Sometimes there is an option on your Chipset that allows you set the amount of system memory you can allocate to your AGP Aperture. I set mine to 256 MB, even though my card only has 128 directly on it. You should notice a difference. For good measure, use your Windows Task Manager and clean out some programs that you dont need running while your gaming. To root out the background programs that you dont need, just enter the file running, like explorer.exe into the field on this website.
It gives you a detailed description of all the .exe files Windows has been known to use for a wide array of programs. It will also telll you if that particular executable is necessary for windows to run. It if isn't, kill it! It may not be your memory, but this will root out that possiblity. Also, spyware and ad ware can hamper your computers capicity to use heavy weight programs like video games. I use Ad-Aware SE Personal and Spysweeper by Webroot. (I download a new trial version of Spysweeper all the time) These are the two most effective anti-spyware/adware programs I have ever used. I have tried many, and these two seem to have alot more definations then the rest, and they are simple to use. Also, if you have a firewall built into your router (as almost all do) you shouldn't have to worry about scanning your computer very often.
Finally, there's the chance of a virus. I use only one website for my virus scanning. They have the most virus definations of any anti-virus software, and their definations are updated DAILY, as opposed to Norton's or McAfee's every two weeks or so. Here is a direct link to the page where you can perform the scan. Its called ActiveScan on www.pandasoftware.com.
Here is the full link. http://www.pandasoftware.com/product..._principal.htm
Make sure to do basic computer maintaince, such as Disk Clean Up, and Defragment. I dont know for sure that any of the above is the cause of your problem, but it should in some way, and improve overall gaming performance, and thats all we really want!
Edit: I forgot to mention, if it was because the card was hot, wouldn't it do a VPU Restore, I believe its called? The computer would restart, in an attempt to save the card from excessive overheating that would cause damage to the card.